As can be seen, the 270 WSM delivers only 49 more f/s but it needs 14.5 more grains of powder to achieve this.
The 7mm WSM actually beats the 7mm Rem. magnum by 19 f/s and needs 5 grains LESS powder to do so.
The .300 Win. Magnum 150 gr. bullet beats the .300 WSM 150 grainer by only 23 f/s, but requires 8.5 gr. more powder to do that.
The .300 Win. Magnum and .300 WSM tie at 3036 f/s with 180 gr. bullets, but the shorty use 3 grains less powder.
To my way of thinking, the 270 WSM (and I own one) is nowhere near as efficient as the .270 Win.
The .300 shorty, (and I own one of these) because it uses less powder than the .300 and almost matches (150 gr.) and matches (180 gr.) the bigger case .300 in velocity, is a pretty efficient round.
The 7mm WSM seems to be the most efficient, beating it's big brother in velocity and using less powder to do it. (I WISH I owned one of these!)

I own several short mags, including the .270 WSM, and handload all of them as well as my other calibers. The short mag factory ammo is terribly expense, and I wouldn't own one if I didn't handload. It seems that the larger caliber short mags are more efficient than the smaller ones. If you check the handloading books, you will find that the 270WSM, for all it's great reputation, is not one of the more efficient shorties. In fact, for the ten grains or more of powder that it takes to load the 270 WSM, the return in gained velocity over the standard, good old 270 Win is very minimal. If you like the .277 class bullets, buy a 270Win. - it does evertything that the 270WSM mag does, but a whole lot cheaper and with less recoil, and you can find ammo everywhere.

The .308 Win. is also a great round. Perhaps a little more recoil than the standard .270 Win, but every bit as accurate, and delivers a little more punch down range. Any deer-size animal, hit in the same place with either of these bullets would be just as dead just as fast!

Don't think I would recomend a .300 to any begining hunter. Too much bang. Trying to kill deer not shoulders.